The present invention relates generally to hydraulic lift systems, and more specifically to a hydraulic lift system including an adjustable valve operable for both hydraulic lift and hydraulic weight transfer functions.
Machines such as, for example, a front-mounted mower utilize both a lift system to raise and lower an implement relative to a vehicle and a mechanical or hydraulic device to transfer weight from the implement to the vehicle during operation. Some machines utilize chains, cables or springs, or a combination thereof, to provide weight transfer. With some front-mounted mower arrangements, an extension spring is attached to the frame of the vehicle and to the implement to transfer implement weight onto the vehicle drive wheels for better traction and to reduce the implement weight on the ground for better flotation. Other structures include a torsion spring mounted on the pivot axis of push arms which are connected to the implement to transfer implement weight to the vehicle drive wheels.
Another weight transfer arrangement which is utilized on presently available John Deere front-mounted mowers includes a weight transfer valve providing an adjustable flow restriction downstream from the operator control valve connected to the hydraulic lift cylinders. By rotating the weight transfer valve, the flow restriction is adjusted to provide a preselected charge pressure to the lift cylinder which, in turn, provides the desired lift for transferring the weight of the implement to the vehicle drive wheels.
The mechanical weight transfer devices require exterior hardware which detracts from the appearance of the vehicle. Mounting brackets are also needed with the mechanical devices, and the implement lift force can vary substantially as the implement goes up and down over the terrain. The adjustment of the lift force with mechanical devices can also be difficult and non-precise.
Although the hydraulic system for transferring weight is relatively more compact and easier to adjust than the mechanical systems, two valves have been necessary to provide both the adjustable weight transfer function and the lift function. The additional valve significantly increases the cost and complexity of the hydraulic system. In addition, presently available weight transfer valves permit the operator to conveniently lift the implement from the ground and maintain the implement in the lifted position rather than following the recommended procedure of maintaining the implement at least partially supported on the ground for increased machine stability. Although the lift or full float position is required on the weight transfer valve to give the operator the capability to climb a curb or small hill or to load the implement onto a trailer, costly ballast is required on the machine to counterbalance the weight of the implement when it is transported in the float position against recommended procedures.